I've read some daft things about "school mum cliques" on this site in my time but this is outstanding. You make it sound as if people are serving 10-15 at Wormwood Scrubs.
In the overwhelming majority of cases these "school mum cliques" exist only in the minds of people with far too much time on their hands who have got the hump because sometime didn't save them a seat at the Christmas concert or invite them for coffee.
Very few people have time to worry about building "cliques" at school, we're too busy worrying about our kids, our jobs and our lives and getting through the day. Some of us, gasp, have pre-existing friendship with mums at school and while that may piss you off, it's a free country.
I've said it before but no one is entitled to make friends with other parents at school: it's not a guaranteed social life on a plate. It's a bonus if it happens, but if you don't make friends it's not some giant conspiracy against you, it's more than likely because people have pre-existing friendships.
Get out and make other friends, find things to distract you, let people get on with their own lives and stop bitching about other parents.